Subscriptions Settings Form

On your subscriptions settings form for a subscription type (like "Categories" or "Groups"), you only list the columns ("Send interval", "On updates", "On comments") for that subscription type if the checkbox is checked.

I'd like to duplicate that form functionality, but can't really understand the code (i.e., "subscriptions_content_type_form()"). I was wondering if you got this code from some other module that I might be able to study to understand how to do this?

Thanks for any assistance on this.